Layer-thickness dependence of the conductive properties of Mo/Si multilayers

Abstract

We report new measurements of the conductance and superconducting transition temperature of a set of Mo/Si multilayers, as a function of the metal layer thickness (from 7--85 \AA{}) for a constant semiconductor layer thickness of 22 \AA{}. Unlike previously reported measurements, we do not observe oscillations in either the resistivity, resistivity ratio, or the superconducting transition temperature with the metal layer thickness. Rather, we observe monotonic variations in the transport properties as the metal layer thickness increases. The sheet conductance and its change between 10 and 300 K both vary approximately linearly with the metal layer thickness, above a threshold thickness. The conductance starts to grow with metal layer thickness at approximately 10 \AA{}, whereas the temperature coefficient of resistance changes sign at approximately 25 \AA{}, exhibiting a Mooij correlation with a crossover resistivity of 125 \ensuremath{\mu}\ensuremath{\Omega} cm. The observed temperature dependence of the conductance rules out localization as the origin of the negative temperature coefficient of resistance. The conductance data are analyzed using a simple phenomenological model involving transport in interfacial and metallic layers, whose relative contribution to the conductance depends on the metal layer thickness and the temperature. The model is applied to separate two competing contributions that determine the overall temperature dependence of the conductance. We attribute the differences between our measurements and previous measurements to differences in bulk metallic conductivities and interface morphologies, due to differences in thermal evaporation versus sputtering fabrication processes. Our results show that the level and nature of disorder is an important ingredient in any theory that explains the cause of the observed oscillations.